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19 presentation on Facial Measurements -
- 1. APPLIED OPTICS Lecture 19 FacialMeasurements - PDs
- 2. PD MEASUREMENT Measuring Principle/Methods •PD rule (visual axes or pupil centres) • Pupillometer (visual axes)
- 3. PD MEASUREMENT PD measurementdevices • PD ruler (monocular/binocular) • Pupillometer • Pen torch
- 4. PD MEASUREMENT Ensure properalignment
- 5. PD MEASUREMENT Reference Points •The corneal reflexes • Pupil centres • Outer edge of the right pupil and inner edge of left • Outer edge of the right iris and the inner edge of the left • Outer canthus of the right eye to the inner canthus of the left
- 6. BINOCULAR DISTANCE PDMEASUREMENT Step 1 Subject Examiner 30 20 10 0 PD Rule 30 40 50 60 20 10 0 PD Rule RE LE RE LE RE LE RE LE 70 Subject Examiner Step 2
- 7. BINOCULAR DISTANCE PD MEASUREMENT Stepsfor measuring binocular distance PD 1. Hold the PD rule against the patient’s forehead and instruct the patient to look into the centre of your open eye (your left eye). 2. Line the zero mark up directly above the reference point on the patient’s right eye (the corneal reflexes formed by a pen torch held directly under the practitioner’s open eye is the best reference). 3. Close your left eye and open your right eye, instructing the patient to look into the centre of that eye. 4. Note the mark directly above the reference point on the patient’s left eye.
- 8. PD MEASUREMENT A pentorch will produce clear corneal reflections which correspond with the visual axes
- 9. BINOCULAR PDS 60 61 The variationbetween the visual axes and pupil centres
- 10. MONOCULAR PD Monocular PDs •Wearers’ faces are not always symmetric • Required for PALs and aspheric single vision (distance and near) 31 34
- 11. MONOCULAR PD RULES •PD rule (visual axes) • Monocular PDs • PD rule positioned on frame and on face
- 12. MEASURING MONOCULAR PDS PDRule and pen torch Method 1. Hold the PD rule with the notch on the patient’s bridge and instruct the patient to look at the pen torch being held directly under the centre of your open eye (your left eye). 2. Read the measurement on the monocular scale directly above the corneal reflex of the light from the pen torch. 3. Close your left eye and open your right eye, instructing the patient to look at the pen torch being held directly under the centre of your right eye. 4. Note the measurement on the monocular scale directly above the corneal reflex on the patient’s left eye.
- 13. Corneal reflex pupillometer(visual axes) CORNEAL REFLEX PUPILLOMETER 61 Monocular PD Monocular PD 32 29
- 14. CORNEAL REFLEX PUPILLOMETER Advantagesof the corneal reflex pupillometer over the PD rule: • Greater consistency as less margin for human error • Considers visual axes as well as pupil centres • Allows for a more accurate measurement of near centration • Looks more professional
- 15. CORNEAL REFLEX PUPILLOMETER Practitioner’s side Subject’s side BinocularPD Monocular PD Fast and accurate measurement of monocular and binocular PDs
- 16. CORNEAL REFLEX PUPILLOMETER Abilityto vary working distance from 20cm to infinity Procedure: • Internal hairline moved until line and corneal reflection are coincident • Corneal reflection typically located nasal to pupil centre (2 to 5) Subject’s view Practitioner’s view
- 17. NEAR CENTRATION DISTANCE Distancebetween where the visual axes cross the spectacle plane when the eyes are observing a near object. • Corneal reflex pupillometer • Calculation • PD rule Near Centration Distance
- 18. MEASURING NEAR CD RELE Spectacle Plane Subject Examiner LE RE 40cm Centre of nose bridge to centre of cornea
- 19. MEASURING NEAR CDUSING PD RULE AND PEN TORCH 1. Using a pen torch, practitioner positioned at near working distance (normally 40 cm) with dominant eye directly in front of patient’s bridge, holding the pen torch underneath their eye toward which the patient is asked to fixate. 2. Using notched PD rule held at spectacle plane, the practitioner reads the measurement (using dominant eye) on monocular scale, which is lined up directly above corneal reflex on patient’s right eye. 3. Finally, without moving, practitioner notes measurement on monocular scale directly above the corneal reflex on patient’s left eye.
- 20. DETERMINING THE NEAR CDUSING FORMULAE • Near centration distance • The calculation Centre of rotation to spectacle plane 27mm Working distance 400 mm Near Centration Distance Distance PD
- 21. PD MEASUREMENT Sources oferror taking PDs with the PD rule • Parallax: practitioner does not close the non-observing eye • Parallax: practitioner and patient are not directly in line • Incorrect ruler position: not straight or close enough to the spectacle plane, especially for near measurement • Significant differences in PDs of subject and examiner
- 22. ASPHERICS AND NEARCD • If aspheric lenses are used for reading spectacles lenses should be fitted on distance PD to ensure that the principal axis passes through the eye’s centre of rotation. • Helps minimise oblique astigmatism • While setting the lenses up according to the patient’s distance PD will provide the best optics, it will create unwanted prism at near. • This prismatic effect will be similar to prismatic effect experienced by the patient when wearing bifocals or progressives but will be significantly less than their fusional convergence reserves
- 23. ASPHERICS AND NEARCD • E.g. Rx = +6.00D OU Add = +1.00D OU Readers set at distance PD = 2.8 base out Readers set at near CD = 0 Prism at near with bifocals = 2.4 base out
- 24. CONCLUSION • Why MeasurePDs? • Spectacle lens optical centres (OCs) should correspond to the wearer’s PD • Advanced lens designs must be positioned accurately • To avoid unwanted prismatic effect from a mismatching of centration distances and pupil centres especially with higher Rx powers • PD measurements are critical for accurate fitting